Spring steel, spring, and manufacturing method of spring

ABSTRACT

A spring steel contains, in terms of mass %, C: 0.35% or more and 0.55% or less, Si: 1.60% or more and 3.00% or less, Mn: 0.20% or more and 1.00% or less, Cr: 0.10% or more and 1.50% or less, Ni: 0.05% or more and 0.30% or less, and Cu: 0.05% or more and 1.00% or less, and substantially does not contain V, the remainder being Fe and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a related application of, and claims priorityto, Japanese Patent Application No. 2013-117874 filed on Jun. 4, 2013,the entire contents of this Japanese patent application are herebyincorporated by reference into the present application.

TECHNICAL FIELD

The present specification relates to a spring steel, a spring, and amanufacturing method of the spring.

BACKGROUND TECHNOLOGY

In recent years, there has been a growing demand for spring steels andsprings having high strength. In Patent Literature 1 to 4, springs aredisclosed that excel in durability and resistance to sag as well assprings that excel in corrosion fatigue strength.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No.    H3-2354-   Patent Literature 2: Japanese Patent Application Laid-Open No.    2008-106365-   Patent Literature 3: Japanese Patent Application Laid-Open No.    2003-55741-   Patent Literature 4: Japanese Patent Application Laid-Open No.    2004-315944-   Patent Literature 5: Japanese Patent Application Laid-Open No.    2004-143482-   Patent Literature 6: Japanese Patent Application Laid-Open No.    2008-202124

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

However, Ni and V are added in all of the springs described in PatentLiterature 1 to 4. Ni is added with the intention of improving corrosionresistance, whereas V is added with the intention of improving toughnessand corrosion fatigue resistance. These springs contain 0.30% or moreand 3.00% or less of Ni and contain 0.05% or more and 0.50% or less ofV, in terms of mass %. Ni and V are costly, and thus the overall cost ofsuch springs tends to be high.

Means for Solving the Problem(s)

As a result of conducting various studies on alloy compositions ofspring steel, the present inventor discovered an alloy composition thatcan ensure the performance of a spring even if the added amount of Ni isreduced and V is not added. The following means are provided by thepresent specification based on these new insights.

In accordance with the present disclosure, a spring steel contains, interms of mass %, C: 0.35% or more and 0.55% or less, Si: 1.60% or moreand 3.00% or less, Mn: 0.20% or more and 1.00% or less, Cr: 0.10% ormore and 1.50% or less, Ni: 0.05% or more and 0.30% or less, and Cu:0.05% or more and 1.00% or less, and substantially does not contain V,the remainder being Fe and unavoidable impurities.

Ni may be 0.18% or more in terms of mass %. Ni may be 0.18% or more and0.25% or less in terms of mass %. Mn may be 0.45% or more and 0.65% orless in terms of mass %. Cu may be 0.20% or more and 0.30% or less interms of mass %. Ti may be 0.030% or more and 0.100% or less in terms ofmass %. B may be 0.0010% or more and 0.0050% or less in terms of mass %.

P may be 0.015% or less and S may be 0.015% or less in terms of mass %.According to the present disclosure, a spring is also provided that ismanufactured using any one of the above-mentioned spring steels.

According to the present disclosure, a manufacturing method of a springusing spring steel is also provided, in which the spring steel contains,in terms of mass %, C: 0.35% or more and 0.55% or less, Si: 1.60% ormore and 3.00% or less, Mn: 0.20% or more and 1.00% or less, Cr: 0.10%or more and 1.50% or less, Ni: 0.05% or more and 0.30% or less, and Cu:0.05% or more and 1.00% or less; and substantially does not contain V,the remainder of the spring steel being Fe and unavoidable impurities.

The manufacturing method may further comprise: a hot setting step inwhich hot setting is performed on the spring steel, which has beenformed into a coil shape and subjected to heat treatment; and a warmshot peening step in which warm shot peening is performed on the springsteel after the hot setting. The manufacturing method of the spring mayfurther comprise: a warm shot peening step in which warm shot peening isperformed on the spring steel, which has been formed into a coil shapeand subjected to heat treatment; and a hot setting step in which hotsetting is performed on the spring steel after the warm shot peening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing results of a durability test.

FIG. 2 is a graph showing results of a clamped sag test.

FIG. 3 is a graph showing results of a corrosion fatigue test.

MODE(S) FOR CARRYING OUT THE INVENTION

According to spring steels disclosed in the present specification(hereinafter referred to as “the present spring steel”), by reducing theadded amount of Ni to 0.05% or more and 0.30% or less, in terms of mass%, and not adding V, it is possible to manufacture relativelyinexpensive springs. Embodiments disclosed in the present specificationwill be explained in detail below. Note that, in the followingexplanation, mass % will be referred to simply as %.

Representative and non-limiting specific examples of the presentdisclosure will be hereinafter explained in detail with reference to thedrawings as appropriate. This detailed explanation is simply intended toshow details for implementing preferred examples of the presentdisclosure to those skilled in the art and is not intended to limit thescope of the present disclosure. Additional features and inventionsdisclosed below may be used separately from or together with otherfeatures and inventions in order to provide a further improved springsteel, a spring, and a manufacturing method of the spring.

Combinations of features and processes disclosed in the followingdetailed explanation are not always essential in implementing thepresent disclosure in a broadest meaning and are described only toexplain, in particular, representative specific examples of the presentdisclosure. Further, in providing additional and useful embodiments ofthe present disclosure, various features of the specific examplesexplained above and below and various features of matters described inindependent and dependent claims do not have to be combined as specifiedin the specific examples described herein or as specified by enumeratedorders.

All features described in the present specification and/or claims areintended to be disclosed individually and independently from oneanother, separately from configurations of features described inexamples and/or claims, as limitations for disclosed or claimed mattersspecifying the invention as originally filed. Further, descriptionsconcerning all numerical value ranges and groups or masses are made withthe intention of disclosing intermediate configurations of the numericalranges and the groups or the masses as limitations for disclosed orclaimed matters specifying the invention as originally filed.

(Spring Steel)

The present spring steel contains, in terms of mass %, C: 0.35% or moreand 0.55% or less, Si: 1.60% or more and 3.00% or less, Mn: 0.20% ormore and 1.00% or less, Cr: 0.10% or more and 1.50% or less, Ni: 0.05%or more and 0.30% or less, and Cu: 0.05% or more and 1.00% or less.

(C: Carbon)

The content of C is preferably 0.35% or more and 0.55% or less. If thecontent of C in the present spring steel is in this range, a springsteel having satisfactory strength can be obtained by quenching andtempering. If the content of C is less than 0.35%, a spring steel havingsatisfactory strength cannot be obtained by quenching and tempering. If,on the other hand, the content of C is more than 0.55%, toughness maydecrease and fatigue strength and corrosion fatigue strength maydecrease. While it depends on the relationship with other alloycomponents, the content of C is preferably 0.40% or more and 0.55% orless. A lower limit value is more preferably 0.43% and still morepreferably is 0.45%. Most preferably, the content of C is 0.45% or moreand 0.53% or less.

(Si: Silicon)

The content of Si is preferably 1.60% or more and 3.00% or less. If thecontent of Si in the present spring steel is in this range, Si iseffective for improvement of resistance to sag, temperingcharacteristics, and corrosion fatigue strength. If the content of Si isless than 1.60%, sufficient effects in this regard are less easilyobtained. If the content of Si is more than 3.00%, decarburization ispromoted during rolling and heat treatment (quenching). In the presentspring steel, from a viewpoint of corrosion fatigue strength and thelike, a lower limit value is more preferably 1.80% and still morepreferably 1.90%. An upper limit value is more preferably 2.50% andstill more preferably 2.05%. Most preferably, the content of Si is 1.90%or more and 2.05% or less.

(Mn: Manganese)

The content of Mn is preferably 0.20% or more and 1.00% or less. If thecontent of Mn is more than 1.00%, toughness tends to decrease. If thecontent of Mn is less than 0.20%, strength and quenchability tend to beinsufficient, and the spring steel tends to crack during rolling. In thepresent spring steel, from the viewpoint of compensating for a decreasedcorrosion fatigue strength caused by the decrease in the Ni addedamount, a lower limit is more preferably 0.30%, still more preferably0.40%, and still more preferably 0.45%. An upper limit value is morepreferably 0.90%, still more preferably 0.80%, still more preferably0.70%, and still more preferably 0.65%. Most preferably, the content ofMn is 0.45% or more and 0.65% or less.

(Cr: Chrome)

The content of Cr is preferably 0.10% or more and 1.50% or less. If thecontent of Cr in the present spring steel is in this range, Cr iseffective for securing strength and improving quenchability. If thecontent of Cr is less than 0.10%, this effect is not providedsufficiently. If the content of Cr is more than 1.50%, the temperedstructure becomes heterogeneous and there is greater susceptibility to arisk of impairing the resistance to sag. An upper limit value is morepreferably 1.0%, still more preferably 0.50%, and still more preferably0.40%. A lower limit value is more preferably 0.20% and still morepreferably 0.25%. Most preferably, the content of Cr is 0.25% or moreand 0.40% or less.

(Ni: Nickel)

The content of Ni is preferably 0.05% or more and 0.30% or less. Ni hasthe effect of improving the toughness of steel, and also has the effectof suppressing corrosion of steel and improving corrosion fatiguestrength. If the content of Ni in the present spring steel is in thisrange, an inexpensive spring is provided as compared to prior springs.An upper limit value is more preferably less than 0.30%, still morepreferably 0.27%, and still more preferably 0.25%. A lower limit valueis more preferably 0.10%, more preferably 0.15%, and more preferably0.18%. Typically, the content of Ni is preferably 0.18% or more and0.30% or less, more preferably 0.18% or more and less than 0.30%, andstill more preferably 0.18% or more and 0.25% or less.

(Cu: Copper)

The content of Cu is preferably 0.05% or more and 1.00% or less. If thecontent of Cu in the present spring steel is in this range, Cu iseffective for improving corrosion resistance and improvingquenchability. If the content of Cu is less than 0.05%, this effect isnot provided sufficiently. If the content of Cu is more than 1.00%,costs increase. In the present spring steel, from these viewpoints, anupper limit value is more preferably 0.70%, still more preferably 0.50%,and still more preferably 0.30%. A lower limit value is more preferably0.10%, still more preferably 0.15%, and still more preferably 0.20%. Cuis preferably 0.10% or more and 0.50% or less and more preferably 0.20%or more and 0.30% or less.

(Ti: Titanium)

The content of Ti is preferably 0.030% or more and 0.100% or less. Ifthe content of Ti in the present spring steel is in this range, prioraustenite crystal grains after quenching are atomized and toughness isimproved. If the content of Ti is less than 0.030%, this effect is notprovided sufficiently. If the content of Ti is more than 0.100%, coarseinclusions may precipitate and toughness may decrease. Further, in thepresent spring steel, for example from the viewpoint that the amount ofNi is reduced and V is substantially not added, an upper limit value ismore preferably 0.095%, still more preferably 0.090%, and still morepreferably 0.085%. A lower limit value is more preferably 0.045% andstill more preferably 0.050%. When the content of Ti is in this range,it is possible to more effectively secure toughness and corrosionfatigue resistance. Most preferably, the content of Ti is 0.050% or moreand 0.095% or less.

(B: Boron)

The content of B is preferably 0.0010% or more and 0.0050% or less. Ifthe content of B in the present spring steel is in this range, ductilityand toughness of a steel wire are improved. If the content of B is lessthan 0.0010%, these effects are not provided sufficiently. If thecontent of B is more than 0.0050%, this effect saturates. Further, inthe present spring steel, for example from the viewpoint that the amountof Ni is reduced and V is substantially not added, an upper limit valueis more preferably 0.0040% and still more preferably 0.0035%. Morepreferably, a lower limit is 0.0013%. Most preferably, the content of Bis 0.0013% or more and 0.0035% or less.

Further, the present spring steel may contain P (phosphorus). Because Ptends to embrittle crystal grain boundaries, the content of P ispreferably 0.015% or less and more preferably 0.010% or less.

The present spring steel may contain S (sulfur). Because, like P, Stends to embrittle crystal grain boundaries, the content of S ispreferably 0.015% or less and more preferably 0.010% or less.

The present spring steel contains the alloy components explained abovebut contains substantially no V. Containing substantially no V meansthat V is included in a same amount as unavoidable impurities.Specifically, the content of V is preferably 0.03% or less. The contentof V is more preferably 0.02% or less, still more preferably 0.01% orless, and still more preferably 0.005% or less. In the present springsteel, the remainder is Fe (iron) together with unavoidable impurities.

(Manufacturing Methods of a Spring)

Methods of manufacturing springs using such spring steel will beexplained next. Various kinds of springs can be manufactured by applyingthe present spring steel, a known hot forming method, a cold formingmethod, a warm forming method, and the like. For example, coil springscan be manufactured as explained below. That is, after the presentspring steel is formed into a round steel bar, a wire material or awire, a plate material, or the like, it is formed into a coil shape.Further, the spring can be manufactured by performing hot setting andwarm shot peening on the coil after the forming. The order of performingthe hot setting and the warm shot peening may be either order. Themethod of performing the warm shot peening after performing the hotsetting will be referred to as a regular order method; the method ofperforming the hot setting after performing the warm shot peening willbe referred to as a reverse order method. By utilizing suchmanufacturing methods, it is possible to obtain coil springs for anautomobile suspension that excel in resistance to sag and durability.

In particular, according to the regular order method, it is possible toobtain a spring that excels in durability. According to the reverseorder method, it is possible to obtain a spring that excels inresistance to sag. That is, the present spring steel can beappropriately used with the regular order method and the reverse ordermethod depending on the use of the spring. In a more specificembodiment, a coil spring for an automobile suspension is manufacturedusing the present spring steel by performing the processes of: forming;heat treatment; hot setting; warm shot peening; cold shot peening; coldsetting; and painting, in this order. The forming process may beperformed in a hot state (at a temperature equal to or higher than therecrystallization temperature of the wire material) or may be performedin a warm state (at a temperature lower than the recrystallizationtemperature of the wire material) or in a cold state (at roomtemperature). Various previously known methods can be used as the methodthat forms the present spring steel into a coil shape. For example, thepresent spring steel may be formed using a coiling machine or may beformed by a method that winds it around a core bar.

In the heat treatment process, heat treatment is performed on the coilthat has been formed into the coil shape by the coil forming process.The heat treatment performed in this process differs depending onwhether the coil forming process is performed in the hot state or in thewarm state or the cold state. That is, when the coil forming process wasperformed in the hot state, quenching and tempering are performed.Strength and toughness are imparted to the coil by the quenching and thetempering. The temperature condition for the quenching can be set to800° C. or more and 1000° C. or less. The temperature condition for thetempering can be set to 300° C. or more and 500° C. or less. On theother hand, when the forming process was performed in the cold state,low-temperature annealing is performed. Harmful residual stresses(tensile residual stresses) in the interior and on the surface of thecoil can be removed by the low-temperature annealing. Thelow-temperature annealing can be performed at a temperature of 300° C.or more and 500° C. or less for 20 minutes or more and 60 minutes orless. The processes of quenching and tempering of the coil andlow-temperature annealing of the coil can be performed according to anypreviously known method.

In the hot setting process, the setting is performed while thetemperature of the coil is in the warm state. The hot setting applies adirectional compressive residual stress to the coil and thus improvesdurability. Further, the resistance of the coil to sag improves with thepresence of relatively large plastic deformations in the coil. Here, thetemperature at which the hot setting is performed can be appropriatelyset within a range of a temperature equal to or lower than therecrystallization temperature of the wire material and higher than roomtemperature. For example, the hot setting of the coil can be performedin a range in which the temperature of the coil is approximately 150° C.or higher and 400° C. or lower. By performing the setting in such atemperature range, it is possible to increase the amount of plasticdeformation imparted to the coil and improve the resistance to sag. Theamount of sag δh of the hot setting can be appropriately determineddepending on the total length L of the coil spring for automobilesuspension (total length Ls at the time of set). Note that, variouspreviously known methods can be used for the setting.

In the warm shot peening process, the coil on which the hot setting hasbeen performed, is subjected to the shot peening in the warm state.Large compressive residual stresses are imparted to the coil surfaceregion by the warm shot peening. Durability and corrosion fatigueresistance of the coil are improved. Here, the temperature at which theshot peening is performed can be appropriately set in a temperaturerange of a temperature equal to or lower than the recrystallizationtemperature of the wire material and higher than room temperature. Forexample, the temperature of the coil can be set to 150° C. or higher and400° C. or lower. Note that, in the regular order method, it isdesirable to perform the warm shot peening at a temperature lower thanthe temperature at which the hot setting is performed because it isunnecessary to heat the coil after the hot setting. On the other hand,in the reverse order method, it is desirable to perform the hot settingat a temperature lower than the temperature at which the warm shotpeening is performed because it is unnecessary to heat the coil afterthe warm shot peening. Various previously known methods can be used as asteel ball shot peening method.

In the cold shot peening process, the shot peening is performed whilethe temperature of the coil is at a normal temperature state. Before thecold shot peening process, the coil needs to be cooled to the normaltemperature; for example, the coil may be cooled by water or may beallowed to stand and cool. By further performing the cold shot peeningin addition to the warm shot peening, it is possible to further improvethe durability of the coil. Note that the diameter of the steel ballsused in the cold shot peening is preferably set smaller than thediameter of the steel balls used in the warm shot peening. For example,when the diameter of the steel balls used in the warm shot peening is1.2 mm, the diameter of the steel balls used in the cold shot peening ispreferably set to 0.8 mm. By performing the warm shot peening and thecold shot peening, large compressive residual stresses are imparted tothe surface region of the coil in the warm shot peening performed first,surface roughness of the coil is improved in the cold shot peeningperformed later, and compressive residual stresses are imparted to thesurface of the coil. As a result, the durability and the corrosionfatigue resistance of the coil are further improved. Note that theconditions of the cold shot peening, that is, projection speed,coverage, projection time, and the number of times of treatment, can beset as appropriate.

In the cold setting process, the setting is performed while thetemperature of the coil is at the normal temperature state. Byperforming the cold setting in addition to the hot setting, theresistance of the coil to sag is further improved. The amount of sag δcof the cold setting can be appropriately determined depending on thetotal length L (the total length Ls at the time of the set) of the coilspring for an automobile suspension. Note that the amount of sag δc ofthe cold setting is preferably smaller than the amount of sag δh of thehot setting. When the cold setting process is finished, the coil surfaceis painted and the coil spring for an automobile suspension iscompleted.

Note that it is also possible to omit the processes of the cold shotpeening and the cold setting and perform only the warm shot peening andthe hot setting. It is also possible to omit the processes of the coldshot peening and the hot setting and perform only the warm shot peeningand the cold setting. It is also possible to omit the processes of thewarm shot peening and the cold setting and perform only the cold shotpeening and the hot setting. It is also possible to omit the processesof the warm shot peening and the hot setting and perform only the coldshot peening and the cold setting. That is, at least one or moreprocesses of the shot peening processes (warm and cold) may be performedand at least one or more processes of the setting processes (hot andcold) may be performed. Processes other than the processes explainedabove also may be included. For example, when the spring steel is formedinto a coil shape by hot forming, heat treatment (quenching) may beperformed before the hot forming. In addition, when the spring steel isformed into a coil shape by cold forming, heat treatment (inductionhardening·tempering) may be performed before the cold forming. Inaddition, a water cooling process may be performed after the hotsetting.

As was explained above, according to the present disclosure, even if theadded amount of Ni is reduced and V is not contained, it is possible toobtain a spring steel and a spring in which performances equal to orhigher than performances of prior spring steels are secured. Such aspring can be suitably used in a coil spring, a leaf spring, a torsionbar spring, a stabilizer bar, and the like used in an automobilesuspension device and the like.

EXAMPLES

Examples in which the present disclosure is realized will be explainedbelow. Note that the examples explained below are specific examples forexplaining the present disclosure and do not limit the presentdisclosure.

1. Durability Test

(1) Preparation of Springs

Springs 1 to 3 were prepared by using sample 1, which is a spring steelhaving the chemical composition shown in Table 1 below; comparativespring 1 was prepared by using comparative sample 1. This manufacturingmethod will be explained. Note that the numbers in Table 1 represent themass % of each of components with respect to the mass of each of thesamples. First, a steel ingot obtained by melting each material in ablast furnace or an electric arc furnace on a scale equivalent to a massproduction was split into slabs and rolled, and thereafter rolled intowire materials.

TABLE 1 (mass %) C Si Mn P S Cr Cu Ni V Ti B Sample 1 0.5 1.99 0.5 0.0050.005 0.28 0.24 0.24 — 0.06 0.002 Comparative 0.49 1.93 0.71 0.011 0.0010.2 — 0.54 0.16 — — sample 1

After an oil tempering treatment was applied to the wire materials ofthe steels of sample 1 and comparative sample 1, cold forming,low-temperature annealing, hot setting, warm shot peening, watercooling, cold shot peening, and cold setting were performed in thisorder (the regular order method) to manufacture springs 1 and 2 andcomparative spring 1. The conditions of the low-temperature annealingwere 430° C. and 20 minutes for spring 1 and were 400° C. and 20 minutesfor spring 2 and comparative spring 1. The temperature of the coilduring the hot setting was set to 330° C. The temperature of the coilduring the warm shot peening was set to 300° C. After the oil temperingtreatment was applied to the wire material of the steel of sample 1,different from the processes of the regular order method, cold forming,low-temperature annealing, warm shot peening, hot setting, watercooling, cold shot peening, and cold setting were performed in thisorder (the reverse order method) to manufacture spring 3. The conditionsof the low-temperature annealing were 400° C. and 20 minutes. Thetemperature of the coil during the warm shot peening was set to 350° C.The temperature of the coil during the hot setting was set to 200° C.

(2) Test Method

In a durability test, the load acting on the coil springs was cyclicallyvaried and the number of oscillating cycles (endurance cycles) wasmeasured until breakage of the coil spring. Here, the stress to beapplied to the coil springs was set to 735 MPa as the average principalstress, the principal stress amplitude was varied, and the number ofendurance cycles was measured. Results of the test are shown in FIG. 1.In FIG. 1, in case the principal stress amplitude is large and thenumber of endurance cycles is large, the durability is high. That is, itcan be said that the durability is high to the extent that the datapoints are located in the upper right of the graph. The data points ofsprings 1 and 2 tend to be located more toward the upper right of thegraph than the data points of comparative spring 1. Therefore, it wasfound that springs 1 and 2 have a higher durability than the durabilityof comparative example 1.

2. Clamped Sag Test

(1) Preparation of the Springs

Springs 1 to 3 and comparative spring 1 were manufactured by a methodthat is the same as the method in the durability test.

(2) Test Method

The clamped sag test is conducted with the coil spring clamped at theheight at the time of maximum load and placed in a constant temperaturevessel for a predetermined time. Changes in load at the mounting heightwere measured before and after the test to calculate the residual shearstrain amount. Here, the clamping stresses were set to 1150 MPa and 1350MPa. The temperature of the constant temperature vessel was set to 80°C. and the test time was set to 96 hours. In addition, in this clampedsag test, two springs were prepared for each spring and the same testwas performed on the respective springs. Results of the test are shownin FIG. 2. As shown in FIG. 2, at the clamping stresses of both 1150 MPaand 1350 MPa, the residual shear strain amount of spring 3 was smallerthan the residual shear strain amount of comparative spring 1. That is,it was found that a spring with high resistance to sag was obtained bythe reverse order method.

3. Corrosion Fatigue Test

(1) Preparation of Springs

First, after an oil tempering treatment was applied to the steels ofsample 1 and comparative sample 1, cold forming and low-temperatureannealing were performed to respectively prepare spring 4 andcomparative spring 2. The spring specifications of spring 4 andcomparative spring 2 are as shown in Table 2.

TABLE 2 Coil Wire average Free Effective Spring diameter diameter lengthnumber of constant (mm) (mm) (mm) turns (N/mm) φ12.0 φ112 370 4.82 45

Further, spring 5 and comparative spring 3 were prepared from sample 2and comparative sample 2 respectively having the chemical compositionsshown in Table 3 below. Their manufacturing method will be explained.Note that the numbers in Table 3 represent the mass % of each of thecomponents with respect to the mass of each of the samples. First, asteel ingot obtained by melting each material in a blast furnace or anelectric furnace in a scale equivalent to a mass production was splitinto slabs and rolled and thereafter rolled into wire materials.

TABLE 3 (mass %) C Si Mn P S Cr Cu Ni V Ti B Sample 2 0.52 2.04 0.610.007 0.006 0.36 0.2 0.19 — 0.059 0.0014 Comparative 0.51 2.01 0.730.008 0.005 0.24 0.01 0.5 0.16 — — sample 2

After an oil tempering treatment was applied to the wire materials ofthe steels of sample 2 and comparative sample 2, cold forming andlow-temperature annealing were performed to manufacture spring 5 andcomparative spring 3. The treatment conditions and treatmenttemperatures were respectively the same as those in the method ofmanufacturing spring 1 and comparative spring 1. Note that the springspecifications of spring 5 and comparative spring 3 are as shown inTable 4.

TABLE 4 Coil Wire average Free Effective Spring diameter diameter lengthnumber of constant (mm) (mm) (mm) turns (N/mm) φ12.0 φ130 450.5 5.2424.3

Pits were artificially formed on each of the obtained springs and afatigue test was conducted in a corrosive environment. The pits wereformed by disposing a mask having small holes on the outer surfaces ofthe springs at a location where the principal stress amplitude is thegreatest (2 turns from the coil end in spring 4 and comparative spring2, and 3.1 turns in spring 5 and comparative spring 3). Further,hemispherical holes (artificial pits) having a diameter of 600 μm and adepth of 300 μm were formed by electrolytic etching. An aqueous ammoniumchloride solution was used as the electrolyte. The corrosive environmentutilized a 5% NaCl water solution as the corrosive liquid and onlyportions having the artificial pits were corroded for 16 hours by amisting apparatus. In addition, the circumferences of the artificial pitportions were covered with adsorbent cotton impregnated with the 5% NaClwater solution, and it was prevented from drying out using an ethylenewrap. The fatigue test was conducted in this wrapped state and thenumber of oscillations until breakage was evaluated. In the fatiguetest, the oscillating rate was set to 2 Hz and excitations were appliedby parallel compression using a flat base. The test heights were setbased upon a principal stress condition of 495±300 MPa that wasdetermined as if there were no artificial pits at locations whereartificial pits were formed. Results are shown in FIG. 3. As shown inFIG. 3, in spring 4 and comparative spring 2, averages of the number ofcorrosion endurance cycles were approximately equal. Therefore,corrosion fatigue strengths of both the springs were approximatelyequal. In spring 5 and comparative spring 3, averages of the number ofcorrosion endurance cycles were approximately equal. Therefore, thecorrosion fatigue strengths of both the springs were approximatelyequal.

From the above, it has been found that the present spring steelmaintains toughness and corrosion fatigue strength, although the contentof Ni is reduced and V is substantially not contained. In addition, withregard to resistance to sag, in the present spring steel, the regularorder method and the reverse order method can be suitably used dependingon the use of the spring.

1. A spring steel containing, in terms of mass %, C: 0.35% or more and0.55% or less, Si: 1.60% or more and 3.00% or less, Mn: 0.20% or moreand 1.00% or less, Cr: 0.10% or more and 1.50% or less, Ni: 0.05% ormore and 0.30% or less, and Cu: 0.05% or more and 1.00% or less, andsubstantially does not contain V, the remainder being Fe and unavoidableimpurities.
 2. The spring steel according to claim 1, wherein Ni is0.18% or more in terms of mass %.
 3. The spring steel according to claim1, wherein Ni is 0.18% or more and 0.25% or less in terms of mass %. 4.The spring steel according to claim 1, wherein Mn is 0.45% or more and0.65% or less in terms of mass %.
 5. The spring steel according to claim1, wherein Cu is 0.20% or more and 0.30% or less in terms of mass %. 6.The spring steel according to claim 1, wherein Ti is 0.030% or more and0.100% or less in terms of mass %.
 7. The spring steel according toclaim 1, wherein B is 0.0010% or more and 0.0050% or less in terms ofmass %.
 8. The spring steel according to claim 1, wherein Cu is 0.10% ormore and 0.50% or less and Ni is 0.18% or more and less than 0.30% interms of mass %.
 9. The spring steel according to claim 1, wherein Cu is0.20% or more and 0.30% or less and Ni is 0.18% or more and 0.25% orless in terms of mass %.
 10. A spring manufactured using the springsteel according to claim
 1. 11. A method for manufacturing a spring,comprising: forming a spring steel into a shape of a spring, wherein thespring steel comprises, in terms of mass %, C: 0.35% or more and 0.55%or less, Si: 1.60% or more and 3.00% or less, Mn: 0.20% or more and1.00% or less, Cr: 0.10% or more and 1.50% or less, Ni: 0.05% or moreand 0.30% or less, and Cu: 0.05% or more and 1.00% or less, andsubstantially does not contain V, and the remainder of the spring steelbeing Fe and unavoidable impurities.
 12. The method according to claim11, further comprising: performing hot setting on the spring steel,which has been formed into a coil shape and subjected to heat treatment;and performing warm shot peening on the spring steel after the hotsetting.
 13. The method according to claim 11, further comprising:performing warm shot peening on the spring steel, which has been formedinto a coil shape and subjected to heat treatment; and performing hotsetting on the spring steel after the warm shot peening.
 14. The methodaccording to claim 11, further comprising: performing low-temperatureannealing at 430° C. for 20 minutes; performing hot setting on thespring steel, which has been formed in a coil shape and subjected toheat treatment; and performing warm shot peening on the spring steelafter the hot setting.
 15. The method according to claim 11, furthercomprising: performing low-temperature annealing at 400° C. for 20minutes; performing hot setting on the spring steel, which has beenformed in a coil shape and subjected to heat treatment; and performingwarm shot peening on the spring steel after the hot setting.
 16. Themethod according to claim 11, further comprising: performing hot settingon the spring steel, which has been formed in a coil shape and subjectedto heat treatment at 150° C. or higher and 400° C. or lower; andperforming warm shot peening on the spring steel after the hot setting.17. The method according to claim 11, further comprising: performing hotsetting on the spring steel, which has been formed in a coil shape andsubjected to heat treatment; and after the hot setting, performing warmshot peening on the spring steel at 150° C. or higher and 400° C. orlower.
 18. The spring steel according to claim 1, wherein Ni is 0.18% ormore and Cu is 0.20% or more and 0.30% or less in terms of mass %. 19.The spring steel according to claim 1, wherein Cu is 0.20% or more and0.30% or less, Ti is 0.030% or more and 0.100% or less and B is 0.0010%or more and 0.0050% or less in terms of mass %.
 20. The spring steelaccording to claim 19, wherein in terms of mass %: C is 0.45% or moreand 0.53% or less, Si is 1.90% or more and 2.05% or less, Mn is 0.45% ormore and 0.65% or less, Cr is 0.25% or more and 0.40% or less, Ni is0.18% or more and 0.25% or less, Ti is 0.050% or more and 0.095% orless, B is 0.0013% or more and 0.0035% or less, P is 0.010% or less, Sis 0.010% or less and V is 0.03% or less.